Many medical assays are in use that require use of blood or other bodily fluid samples obtained from patients by a penetrating medical implement such as a needle or cannula. In particular, diabetes patients and patients on anticoaguation therapies must frequently obtain blood samples for use in medical assays. The blood sampling devices typically involve a needle that has a small channel along which body fluid can travel once the skin is punctured by the needle. The bodily fluid sample thus obtained is applied to a test strip, introduced to a reader or sensor device, or otherwise used in an assay to monitor glucose levels, levels of therapeutic compounds present in the fluid, or other properties of the fluid.
In order to facilitate quick movement of the fluid along the channel without using a vacuum or pressure source or other external means, the surface of the needle needs to possess low contact angle property with respect to the bodily fluid. The materials from which needles can be inexpensively fabricated are tough plastics and metals. Unfortunately, the surface properties of these materials tend to be hydrophobic and do not promote capillary flow of bodily fluid through the needle.
Hydrophilic polymers, such as poly(2-hydroxyethyl methacrylate) were originally developed for use in soft, hydrophilic contact lenses and for use in the controlled release of drugs in drug delivery systems. Acrylic hydrogel polymers of this sort have been more recently been used to improve lubricity of needles during insertion into the human body. In many instances, however, a primer coating or other surface pretreatment has to be completed on a needle prior to coating the needle with a hydrophilic material such as an acrylic hydrogel. Surface pretreatment typically involves either corona discharge or plasma etching, which requires expensive equipment. This has made the use of hydrophilic coatings on needles unattractive. In other cases, crosslinking groups are introduced to the polymer coating to affect the permeability and mechanical properties of the coating. However, crosslinking of the coating frequently requires use of more complex formulation and/or use of expensive energy sources, such as a high intensity UV lamp or electron beam generator, to provide cross-linking in a previously formed coating.
U.S. Pat. No. 4,987,182 describes a method for applying a hydrophilic coating which uses a toxic and flammable solvent mixture containing isopropanol, methyl ethyl ketone and diacetone alcohol to deposit a hydrophilic coating consisting of poly(vinylbutyral) and poly(vinylpyrrolidone). Evaporation of toxic organic solvent into the atmosphere can raise environmental as well as health and safety concerns.
Polyurethane aqueous dispersions are also known as hydrophilic coating materials. However, the polymer must be highly purified in order to rid itself of any sensitizing monomeric isocyanates that may be present in the polymer. It is desirable that coatings for medical implements which come in contact with the skin to be non-allergenic and cause minimal irritation or sensitization to the skin, and urethane-based coatings have been generally unable to provide this feature.
U.S. Pat. No. 5,509,899 describes the use of poly(alkylene glycol) as a second, anti-blocking coating on top of a first hydrophilic coating. The secondary coating prevents adherence of adjacent coated devices such as catheters from adhering to each other upon contact. Unfortunately, the application of secondary, tertiary and higher numbers of coatings complicates and increases the expense of the overall coating process for medical implements, making this approach unfeasible for inexpensive needles.
Thus there is a need for a hydrophilic coating for medical devices that is non-toxic, that is easily applied in a single coating, that is very wettable and compatible to biological fluids, and is simple and inexpensive to implement. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
Relevant Literature:
U.S. Pat. No. 5,509,899; U.S. Pat. No. 4,987,182; U.S. Pat. No. 3,734,874; U.S. Pat. No. 5,925,336; U.S. Pat. No. 6,001,910.